Sunday, October 09, 2011

Tennessee BBS: Ups and Downs

I have heard and read comments in the recent past suggesting that in another 50 years our North American avifauna will consist of a highly depauerate relict of exotics and a few generalists, with most of our native birds religated to isolated remainders if not outright extinction. If this doomsday future is going to come to pass, it certainly is not giving any hints of itself yet in Tennessee. These 45 years of Breeding Bird Survey data for Tennessee actually paint a picture of an avifauna that remains robust, diverse, and overwhelmingly native. Though there are some major red flags, and one native species that has been effectively extirpated in recent decades, the majority of Tennessee's breeding bird species have shown statistically significant increases over this time period. Exotics are not taking over the landscape; while there have been two new arrivals during the time frame, of the three other long establish invaders two have shown flat numbers while the third has declined rather drastically. The median for the 104 species that have sufficient data to establish a meaningful trend is a 66% increase between the 1966-1975 interval and the 2001-2011 period. This looks like a sign of Apocalypse Not rather than Apocalypse Now.

Downs

As mentioned, there are some "red flag" species showing ongoing major declines in numbers, and more that have experienced smaller drops. So to get to the bad news first, these 27 species showed statistically significant declines in BBS counts (biggest loosers listed first):

In this list and all the remaining analyses, I have not included species that occur only on small numbers of routes (generally high-elevation birds of far east Tennessee or wetland birds of far west Tennessee) or that were not recorded in enough years to display a meaningful trend. Many of these species are ones whose declines have attracted notice and concern, such as Bewick's Wren, Loggerhead Shrike, and Northern Bobwhite. But some were surprises to me. How much concern have you heard expressed about the disappearance of Northern Flickers as a breeding species in the region? Or Gray Catbirds? Or Orchard Orioles? It is interesting to note that the majority of these species are to a fairly large degree now dependent on human activities to create and maintain their habitats -- from the obvious (House Sparrows, Chimney Swifts) to the more subtle (Thrashers in early successional habitats, Belted Kingfishers on farm ponds). I will discuss habitat and landscape-level patterns in these trends more in a forthcoming post.

Holding Steady

These next 17 species showed trends that, though no identically equal to zero, where not significantly different from it. Again they are listed in order of their percentage change, even though these changes were not statistically significant:

Though this is also a diverse list, two patterns jump out as you read through it. First, as I mentioned before, this list includes all of the diurnal birds of prey plus the two vultures. Even American Kestrels, subject of much concern regionally, showed a positive trend. Though it is just hypothesizing on my part, I have to suspect that this trend must be a combination of the effects of the DDT ban and reduced persecution because of better laws, law enforcement, and awareness. I know my own rural neighbors do not routinely shoot hawks; I suspect this would not have been the case 45 years ago!

The second pattern is a larger one which I will discuss more in the next post: Most of our forest birds are on this list. As Tennessee is principally a land of forests, both at present and in our ecological past, this pattern is the primary contributor to the overall message of good news for Tennessee's breeding birds coming out of these data. We still are home to extensive forests that are some of the most biodiverse temperate ecosystems on the planet, and our avifauna continues to strongly reflect this.